I have never been able to confirm the term “dachi” or “tachi”. Both of which you might see for stances.
“体勢 【たいせい】 (n) posture; stance; (P)” (copied from http://nihongo.monash.edu/cgi-bin/wwwjdic?1E). This is pronounced “Taisei” (tie-say) though, so I don’t know that it fits.
“立ち居; 起ち居; 立居; 起居(io) 【たちい】 (n) movements; sitting and standing” (copied from http://nihongo.monash.edu/cgi-bin/wwwjdic?1F), pronounced “tachii” (tah-chee), could be a better origin for the term. It would imply that the positions aren’t static.
Stance videos below! :oD